In the manufacture of cement using a rotary cement kiln, it is known to position a suspension type preheater contiguous the feed end of the kiln and to direct the exhaust gases from the rotary kiln through the suspension type preheater in countercurrent flow direction with respect to the flow of the feed or "meal" which is in finely divided particulate form, whereby to preheat the meal and to decarbonize a portion of certain constituents such as calcium carbonate (CaCO.sub.3) and magnesium carbonate (MgCO.sub.3) prior to the entry of the meal into the kiln. In the case of a cement kiln, the "meal" also contains compounds of silicon, iron, and aluminum, all of which undergo various chemical reactions during the course of the heat treatment process in the rotary kiln into which the "meal" is discharged from the suspension preheater. The end product produced by the heat treatment process in the kiln is cement.
Examples of prior art patents showing use of suspension type preheaters for use in the heat treatment of finely subdivided particulate material include the following U.S. Pat. Nos. 3,265,775 issued to Wolfgang Friedrich on Aug. 9, 1966; 3,364,583 issued to Wolfgang Friedrich on Jan. 23, 1968; and 4,063,875 issued to Masaaki Takeuchi on Dec. 20, 1977.
Several problems have been encountered in the operation of suspension type preheaters of the general type disclosed by the aforementioned U.S. Pat. Nos. 3,265,775 and 3,364,583 to Wolfgang Friedrich. These problems may be enumerated as follows:
(1) In a suspension preheater constructed in accordance with the principles of the aforementioned two prior art Friedrich patents, the preheated meal is discharged from the lowermost twin cyclones (i.e.--third stage cyclones) into the interior of the fourth-stage preheating chamber such as a conical shaft at a location approximately at the mid-point of the height of the fourth-stage conical shaft. The "meal" thus discharged into the fourth-stage conical shaft is entrained by high velocity exhaust gas from the rotary kiln, which gas is passing upwardly through the fourth-stage conical shaft in countercurrent flow relation to the inlet meal which is discharging downwardly from the third stage cyclones. A relatively large quantity of the powder-like meal (in fact, a relatively large oversupply of the "meal") must be supplied to and maintained in the fourth-stage conical shaft to overcome the upward thrust of the high velocity exhaust gas and to thus establish a meal flow out of the bottom of the fourth-stage conical shaft. This, in turn, causes an undesirably high degree of gaseous entrainment of the "meal" in the lower portion of the fourth-stage conical shaft and also in the feed end housing of the rotary kiln to which the lower end of the fourth-stage conical shaft is connected. The various factors just described produce an undesirable recirculation of the "meal" between the feed end housing and the suspension preheater and between the fourth-stage conical shaft and the third-stage cyclones, causing an undesirable high pressure drop across the suspension type preheater. This high pressure drop of the kiln gas across the preheater requires use of a higher pressure exhaust fan, with consequent greater capital investment and greater expenditure of energy. This recirculation of the meal also increases the preheater offgas temperature thereby causing undesirable thermal losses.
(2) If the prior art Friedrich preheater is used in conjunction with a system in which a portion of the kiln exhaust gas is extracted or bypassed from the kiln feed end housing for control of the alkali level of the product clinker, the high degree of meal concentration in the kiln exhaust gas at the point of the bypass extraction causes a large quantity of meal to be extracted along with the bypass gas. The loss of this feed represents an economic penalty associated with bypass operation when using a suspension type preheater arrangement of the type shown in the aforementioned United States patents to Friedrich.
(3) It has been known in connection with suspension preheaters of the type shown by the aforementioned Friedrich patents to provide a fuel burner on the interior of the preheating chamber such as the hollow conical shaft at the lower portion of the suspension preheater for the purpose of increasing the level of calcination of the meal, with consequent increased cement clinker production. However, this has not proven to be successful in the case of suspension preheaters of the prior art Friedrich type due to the fact that the high concentration of "meal" in the conical shaft has provided a very poor environment and atmosphere for combustion and therefore, to the best of our knowledge, prior art attempts to employ a fuel burner in the conical shaft of prior art Friedrich preheaters have been abandoned.